State-of-the-art instruments for detecting extraterrestrial life

Can Chirality Answer Whether We Are Alone?

Detecting biosignatures of life in extraterrestrial environments remains one of the primary objectives of scientific inquiry. Currently, both remote and direct detection methods are primarily aimed at identifying key molecular classes fundamental to terrestrial biology. However, a more universally applicable spectroscopic approach could involve searching for homochiral molecules. Thus, this perspective delves into the significance of homochirality as a critical factor in the origin of life. Without homochirality, the formation of self-recognizing and self-replicating complex molecules would be hindered. The various hypotheses concerning the origin of homochiral molecules have been explored and analyzed within this context. This perspective emphasizes the potential for discovering extraterrestrial microscopic life through the detection of homochiral molecules using chirodetecting methods such as chromatography and chiroptical spectroscopy or circular polarimetry as a promising remote technique. This discussion highlights the importance of homochirality in the broader search for life beyond Earth and underscores the need for innovative methodologies and instrumentation in astrobiological research. These techniques can be an effective method for detecting homochirality on future planetary missions.

Planetary Minerals Catalyze Conversion of a Polycyclic Aromatic Hydrocarbon to a Prebiotic Quinone: Implications for Origins of Life

Polycyclic aromatic hydrocarbons (PAHs) are ubiquitous in astrochemical environments and are disbursed into planetary environments via meteorites and extraterrestrial infall where they may interact with mineral phases to produce quinones important for origins of life. In this study, we assessed the potential of the phyllosilicates montmorillonite (MONT) and kaolinite (KAO), and the enhanced Mojave Mars Simulant (MMS) to convert the PAH anthracene (ANTH) to the biologically important 9,10-anthraquinone (ANTHQ). All studied mineral substrates mediate conversion over the temperature range assessed (25-500°C). Apparent rate curves for conversion were sigmoidal for MONT and KAO, but quadratic for MMS. Conversion efficiency maxima for ANTHQ were 3.06% ± 0.42%, 1.15% ± 0.13%, and 0.56% ± 0.039% for MONT, KAO, and MMS, respectively. We hypothesized that differential substrate binding and compound loss account for the apparent conversion kinetics observed. Apparent loss rate curves for ANTH and ANTHQ were exponential for all substrates, suggesting a pathway for wide distribution of both compounds in warmer prebiotic environments. These findings improve upon our previously reported ANTHQ conversion efficiency on MONT and provide support for a plausible scenario in which PAH-mineral interactions could have produced prebiotically relevant quinones in early Earth environments.

Assessment of Automated Nucleic Acid Extraction Systems in Combination with MinION Sequencing As Potential Tools for the Detection of Microbial Biosignatures

The utilization of nanopore technologies for the detection of organic biogenic compounds has garnered significant focus in recent years. Oxford Nanopore Technologies' (ONT) MinION instrument, which can detect and sequence nucleic acids (NAs), is one such example. These technologies have much promise for unambiguous life detection but require significant development in terms of methods for extraction and preparation of NAs for biosignature detection and their feasibility for use in astrobiology-focused field missions. In this study, we tested pre-existing, automated, or semiautomated NA extraction technologies, coupled with automated ONT VolTRAX NA sample preparation, and verification with Nanopore MinION sequencing. All of the extraction systems tested (SuperFastPrep2, ClaremontX1, and SOLID-Sample Preparation Unit) showed potential for extracting DNA from Canadian High Arctic environments analogous to Mars, Europa, and Enceladus, which could subsequently be detected and sequenced with the MinION. However, they differed with regard to efficacy, yield, purity, and sequencing and annotation quality. Overall, bead beating-based systems performed the best for these parameters. In addition, we showed that the MinION could sequence unpurified DNA contained in crude cell lysates. This is valuable from an astrobiology perspective because purification steps are time-consuming and complicate the requirements for an automated extraction and life detection system. Our results indicate that semiautomated NA extraction and preparation technologies hold much promise, and with increased optimization and automation could be coupled to a larger platform incorporating nanopore detection and sequencing of NAs for life detection applications.

Extant Earthly Microbial Mats and Microbialites as Models for Exploration of Life in Extraterrestrial Mat Worlds

As we expand the search for life beyond Earth, a water-dominated planet, we turn our eyes to other aquatic worlds. Microbial life found in Earth's many extreme habitats are considered useful analogs to life forms we are likely to find in extraterrestrial bodies of water. Modern-day benthic microbial mats inhabiting the low-oxygen, high-sulfur submerged sinkholes of temperate Lake Huron (Michigan, USA) and microbialites inhabiting the shallow, high-carbonate waters of subtropical Laguna Bacalar (Yucatan Peninsula, Mexico) serve as potential working models for exploration of extraterrestrial life. In Lake Huron, delicate mats comprising motile filaments of purple-pigmented cyanobacteria capable of oxygenic and anoxygenic photosynthesis and pigment-free chemosynthetic sulfur-oxidizing bacteria lie atop soft, organic-rich sediments. In Laguna Bacalar, lithification by cyanobacteria forms massive carbonate reef structures along the shoreline. Herein, we document studies of these two distinct earthly microbial mat ecosystems and ponder how similar or modified methods of study (e.g., robotics) would be applicable to prospective mat worlds in other planets and their moons (e.g., subsurface Mars and under-ice oceans of Europa). Further studies of modern-day microbial mat and microbialite ecosystems can add to the knowledge of Earth's biodiversity and guide the search for life in extraterrestrial hydrospheres.

Evaluating the Microbial Habitability of Rogue Planets and Proposing Speculative Scenarios on How They Might Act as Vectors for Panspermia

There are two types of rogue planets, sub-brown dwarfs and “rocky” rogue planets. Sub-brown dwarfs are unlikely to be habitable or even host life, but rocky rogue planets may have a liquid ocean under a thick atmosphere or an ice layer. If they are overlain by an insulating ice layer, they are also referred to as Steppenwolf planets. However, given the poor detectability of rocky rogue planets, there is still no direct evidence of the presence of water or ice on them. Here we discuss the possibility that these types of rogue planets could harbor unicellular organisms, conceivably based on a variety of different energy sources, including chemical, osmotic, thermal, and luminous energy. Further, given the theoretically predicted high number of rogue planets in the galaxy, we speculate that rogue planets could serve as a source for galactic panspermia, transferring life to other planetary systems.

The Astrobiology of Alien Worlds: Known and Unknown Forms of Life

Most definitions of life assume that, at a minimum, life is a physical form of matter distinct from its environment at a lower state of entropy than its surroundings, using energy from the environment for internal maintenance and activity, and capable of autonomous reproduction. These assumptions cover all of life as we know it, though more exotic entities can be envisioned, including organic forms with novel biochemistries, dynamic inorganic matter, and self-replicating machines. The probability that any particular form of life will be found on another planetary body depends on the nature and history of that alien world. So the biospheres would likely be very different on a rocky planet with an ice-covered global ocean, a barren planet devoid of surface liquid, a frigid world with abundant liquid hydrocarbons, on a rogue planet independent of a host star, on a tidally locked planet, on super-Earths, or in long-lived clouds in dense atmospheres. While life at least in microbial form is probably pervasive if rare throughout the Universe, and technologically advanced life is likely much rarer, the chance that an alternative form of life, though not intelligent life, could exist and be detected within our Solar System is a distinct possibility.

Habitability of Mars: How Welcoming Are the Surface and Subsurface to Life on the Red Planet?

Mars is a planet of great interest in the search for signatures of past or present life beyond Earth. The years of research, and more advanced instrumentation, have yielded a lot of evidence which may be considered by the scientific community as proof of past or present habitability of Mars. Recent discoveries including seasonal methane releases and a subglacial lake are exciting, yet challenging findings. Concurrently, laboratory and environmental studies on the limits of microbial life in extreme environments on Earth broaden our knowledge of the possibility of Mars habitability. In this review, we aim to: (1) Discuss the characteristics of the Martian surface and subsurface that may be conducive to habitability either in the past or at present; (2) discuss laboratory-based studies on Earth that provide us with discoveries on the limits of life; and (3) summarize the current state of knowledge in terms of direction for future research.

Survival, DNA Integrity, and Ultrastructural Damage in Antarctic Cryptoendolithic Eukaryotic Microorganisms Exposed to Ionizing Radiation

Life dispersal between planets, planetary protection, and the search for biosignatures are main topics in astrobiology. Under the umbrella of the STARLIFE project, three Antarctic endolithic microorganisms, the melanized fungus Cryomyces antarcticus CCFEE 515, a hyaline strain of Umbilicaria sp. (CCFEE 6113, lichenized fungus), and a Stichococcus sp. strain (C45A, green alga), were exposed to high doses of space-relevant gamma radiation (⁶⁰Co), up to 117.07 kGy. After irradiation survival, DNA integrity and ultrastructural damage were tested. The first was assessed by clonogenic test; viability and dose responses were reasonably described by the linear-quadratic formalism. DNA integrity was evaluated by PCR, and ultrastructural damage was observed by transmission electron microscopy. The most resistant among the tested organisms was C. antarcticus both in terms of colony formation and DNA preservation. Besides, results clearly demonstrate that DNA was well detectable in all the tested organisms even when microorganisms were dead. This high resistance provides support for the use of DNA as a possible biosignature during the next exploration campaigns. Implication in planetary protection and contamination during long-term space travel are put forward. Key Words: Biosignatures-Ionizing radiation-DNA integrity-Eukaryotic microorganisms-Fingerprinting-Mars exploration. Astrobiology 17, 126-135.

Norvaline and norleucine may have been more abundant protein components during early stages of cell evolution

The absence of the hydrophobic norvaline and norleucine in the inventory of protein amino acids is readdressed. The well-documented intracellular accumulation of these two amino acids results from the low-substrate specificity of the branched-chain amino acid biosynthetic enzymes that act over a number of related α-ketoacids. The lack of absolute substrate specificity of leucyl-tRNA synthase leads to a mischarged norvalyl-tRNA(Leu) that evades the translational proofreading activities and produces norvaline-containing proteins, (cf. Apostol et al. J Biol Chem 272:28980-28988, 1997). A similar situation explains the presence of minute but detectable amounts of norleucine in place of methionine. Since with few exceptions both leucine and methionine are rarely found in the catalytic sites of most enzymes, their substitution by norvaline and norleucine, respectively, would have not been strongly hindered in small structurally simple catalytic polypeptides during the early stages of biological evolution. The report that down-shifts of free oxygen lead to high levels of intracellular accumulation of pyruvate and the subsequent biosynthesis of norvaline (Soini et al. Microb Cell Factories 7:30, 2008) demonstrates the biochemical and metabolic consequences of the development of a highly oxidizing environment. The results discussed here also suggest that a broader definition of biomarkers in the search for extraterrestrial life may be required.

Isolation of nucleic acids and cultures from fossil ice and permafrost

Owing to their constant low temperatures, glacial ice and permafrost might contain the oldest nucleic acids and microbial cells on Earth, which could prove key to reconstructing past ecosystems and for the planning of missions to other planets. However, recent claims concerning viable cells and microbial nucleic acids obtained from ice- and permafrost cores from hundreds of thousands to millions of years old are not properly authenticated and the findings could be the result of contamination. Here, we discuss the processes that restrict the long-term survival of DNA and/or RNA molecules in ice and permafrost, and highlight sources of contamination that could result in false claims. Additionally, we present a set of precautions, controls and criteria to help ensure that future cultures and sequences are authentic.

Are We from Outer Space?

The biological record suggests that life on Earth arose as soon as conditions were favorable, which indicates that life either originated quickly, or arrived from elsewhere to seed Earth. Experimental research under the theme of “astrobiology” has produced data that some view as strong evidence for the second possibility, known as the panspermia hypothesis. While it is not unreasonable to consider the possibility that Earth’s life originated elsewhere and potentially much earlier, we conclude that the current literature offers no definitive evidence to support this hypothesis.

Chladni’s view, that they fall from the skies, pronounced in 1795, was ridiculed by the learned men of the times. (Rachel, 1881)

Evidence of life on Mars, even if only in the distant past, would finally answer the age-old question of whether living beings on Earth are alone in the universe. The magnitude of such a discovery is illustrated by President Bill Clinton’s appearance at a 1996 press conference to announce that proof had been found at last. A meteorite chipped from the surface of the Red Planet some 15 million years ago appeared to contain the fossil remains of tiny life-forms that indicated life had once existed on Mars. (Young and Martel, 2010)

Origin and evolution of the ribosome

The modern ribosome was largely formed at the time of the last common ancestor, LUCA. Hence its earliest origins likely lie in the RNA world. Central to its development were RNAs that spawned the modern tRNAs and a symmetrical region deep within the large ribosomal RNA, (rRNA), where the peptidyl transferase reaction occurs. To understand pre-LUCA developments, it is argued that events that are coupled in time are especially useful if one can infer a likely order in which they occurred. Using such timing events, the relative age of various proteins and individual regions within the large rRNA are inferred. An examination of the properties of modern ribosomes strongly suggests that the initial peptides made by the primitive ribosomes were likely enriched for l-amino acids, but did not completely exclude d-amino acids. This has implications for the nature of peptides made by the first ribosomes. From the perspective of ribosome origins, the immediate question regarding coding is when did it arise rather than how did the assignments evolve. The modern ribosome is very dynamic with tRNAs moving in and out and the mRNA moving relative to the ribosome. These movements may have become possible as a result of the addition of a template to hold the tRNAs. That template would subsequently become the mRNA, thereby allowing the evolution of the code and making an RNA genome useful. Finally, a highly speculative timeline of major events in ribosome history is presented and possible future directions discussed.

Astrobiological benefits of human space exploration

An ambitious program of human space exploration, such as that envisaged in the Global Exploration Strategy and considered in the Augustine Commission report, will help advance the core aims of astrobiology in multiple ways. In particular, a human exploration program will confer significant benefits in the following areas: (i) the exploitation of the lunar geological record to elucidate conditions on early Earth; (ii) the detailed study of near-Earth objects for clues relating to the formation of the Solar System; (iii) the search for evidence of past or present life on Mars; (iv) the provision of a heavy-lift launch capacity that will facilitate exploration of the outer Solar System; and (v) the construction and maintenance of sophisticated space-based astronomical tools for the study of extrasolar planetary systems. In all these areas a human presence in space, and especially on planetary surfaces, will yield a net scientific benefit over what can plausibly be achieved by autonomous robotic systems. A number of policy implications follow from these conclusions, which are also briefly considered.

The search for alien life in our solar system: strategies and priorities

With the assumption that future attempts to explore our Solar System for life will be limited by economic constraints, we have formulated a series of principles to guide future searches: (1) the discovery of life that has originated independently of our own would have greater significance than evidence for panspermia; (2) an unambiguous identification of living beings (or the fully preserved, intact remains of such beings) is more desirable than the discovery of markers or fossils that would inform us of the presence of life but not its composition; (3) we should initially seek carbon-based life that employs a set of monomers and polymers substantially different than our own, which would effectively balance the need for ease of detection with that of establishing a separate origin; (4) a "follow-the-carbon" strategy appears optimal for locating such alternative carbon-based life. In following this agenda, we judge that an intensive investigation of a small number of bodies in our Solar System is more likely to succeed than a broad-based survey of a great number of worlds. Our priority for investigation is (1) Titan, (2) Mars, (3) Europa. Titan displays a rich organic chemistry and offers several alternative possibilities for the discovery of extant life or the early stages that lead to life. Mars has already been subjected to considerable study through landers and orbiters. Although only small amounts of methane testify to the inventory of reduced carbon on the planet, a number of other indicators suggest that the presence of microbial life is a possibility. Care will be needed, of course, to distinguish indigenous life from that which may have spread by panspermia. Europa appears to contain a subsurface ocean with the possibility of hydrothermal vents as an energy source. Its inventory of organic carbon is not yet known.

The Urey instrument: an advanced in situ organic and oxidant detector for Mars exploration

The Urey organic and oxidant detector consists of a suite of instruments designed to search for several classes of organic molecules in the martian regolith and ascertain whether these compounds were produced by biotic or abiotic processes using chirality measurements. These experiments will also determine the chemical stability of organic molecules within the host regolith based on the presence and chemical reactivity of surface and atmospheric oxidants. Urey has been selected for the Pasteur payload on the European Space Agency's (ESA's) upcoming 2013 ExoMars rover mission. The diverse and effective capabilities of Urey make it an integral part of the payload and will help to achieve a large portion of the mission's primary scientific objective: "to search for signs of past and present life on Mars." This instrument is named in honor of Harold Urey for his seminal contributions to the fields of cosmochemistry and the origin of life.

Biochemistry of amino acid racemization and clinical application to musculoskeletal disease

During aging, proteins are subject to numerous forms of damage. Several types of non-enzymatic post-translational modifications have been described in aging proteins, including oxidation, nitration, glycation, and racemization. Racemization of amino acids is the spontaneous conversion of L-enantiomers to the D-form, which is dependent on temperature, pH, and time. Because of the time-dependent nature of racemization, it can be used to determine the relative age and turnover rates of long-lived proteins. There are many such long-lived proteins within the body; they are found in the brain, eye, and heart, but are particularly abundant in proteins found in musculoskeletal tissues such as bone and cartilage. During disease, musculoskeletal tissues have pathologically altered turnover rates. Because turnover rates can be estimated from levels of racemization, racemized musculoskeletal protein fragments may serve as useful biomarkers of disease. This review discusses the biochemistry of amino acid racemization in proteins and its clinical application to musculoskeletal disease.

Survival and germinability of Bacillus subtilis spores exposed to simulated Mars solar radiation: implications for life detection and planetary protection

Bacterial spores have been considered as microbial life that could survive interplanetary transport by natural impact processes or human spaceflight activity. Deposition of terrestrial microbes or their biosignature molecules onto the surface of Mars could negatively impact life detection experiments and planetary protection measures. Simulated Mars solar radiation, particularly the ultraviolet component, has been shown to reduce spore viability, but its effect on spore germination and resulting production of biosignature molecules has not been explored. We examined the survival and germinability of Bacillus subtilis spores exposed to simulated martian conditions that include solar radiation. Spores of B. subtilis that contain luciferase resulting from expression of an sspB-luxAB gene fusion were deposited on aluminum coupons to simulate deposition on spacecraft surfaces and exposed to simulated Mars atmosphere and solar radiation. The equivalent of 42 min of simulated Mars solar radiation exposure reduced spore viability by nearly 3 logs, while germination-induced bioluminescence, a measure of germination metabolism, was reduced by less than 1 log. The data indicate that spores can retain the potential to initiate germination-associated metabolic processes and produce biological signature molecules after being rendered nonviable by exposure to Mars solar radiation.

Development and evaluation of a microdevice for amino acid biomarker detection and analysis on Mars

The Mars Organic Analyzer (MOA), a microfabricated capillary electrophoresis (CE) instrument for sensitive amino acid biomarker analysis, has been developed and evaluated. The microdevice consists of a four-wafer sandwich combining glass CE separation channels, microfabricated pneumatic membrane valves and pumps, and a nanoliter fluidic network. The portable MOA instrument integrates high voltage CE power supplies, pneumatic controls, and fluorescence detection optics necessary for field operation. The amino acid concentration sensitivities range from micromolar to 0.1 nM, corresponding to part-per-trillion sensitivity. The MOA was first used in the lab to analyze soil extracts from the Atacama Desert, Chile, detecting amino acids ranging from 10-600 parts per billion. Field tests of the MOA in the Panoche Valley, CA, successfully detected amino acids at 70 parts per trillion to 100 parts per billion in jarosite, a sulfate-rich mineral associated with liquid water that was recently detected on Mars. These results demonstrate the feasibility of using the MOA to perform sensitive in situ amino acid biomarker analysis on soil samples representative of a Mars-like environment.

D-Amino acids and D-Tyr-tRNA(Tyr) deacylase: stereospecificity of the translation machine revisited

Until 30 years ago, it had been considered that D-amino acids were excluded from living systems except for D-amino acids in the cell wall of microorganisms. However, D-amino acids, in the form of free amino acids, peptides and proteins, were recently detected in various living organisms from bacteria to mammals. The extensive distribution of bio-functional D-amino acids challenges the current concept of protein synthesis: more attention should be paid to the stereospecificity of the translation machine. Besides aminoacyl-tRNA synthetases, elongation factor Tu and some other mechanisms, D-Tyr-tRNA(Tyr) deacylases provide a novel checkpoint since they specifically recycle misaminoacylated D-Tyr-tRNA(Tyr) and some other D-aminoacyl-tRNAs. Their unique structure represents a new class of tRNA-dependent hydrolase. These unexpected findings have far-reaching implications for our understanding of protein synthesis and its origin.

Adenine and RNA in mineral samples. Surface-enhanced Raman spectroscopy (SERS) for picomole detections

Studies on the interactions of biological macromolecules with mineral surfaces are crucial for the detecting biomarkers. But before this can be done for real samples like rocks or sediments, rational methods based on mineral models plus known amounts of nucleic acids must be developed. The methods must be very sensitive, as the amount of bound macromolecule may be very small. Surface-enhanced Raman spectroscopy (SERS) is perfect for detecting picomolar amounts of nucleic acid materials. In this study, the models used were adenine and GAAA hairpin for nucleic acids materials and a clay (montmorillonite) plus colloidal silver (used for SERS detection) for mineral supports. We have shown that OH(-) anions compete with adenine and the adenyl residues in the GAAA loop for adsorption onto nano-sized silver particles in basic medium. The GAAA adenyl moieties are less well adsorbed onto either clay or silver than is adenine. Also, the transfer of either adenine or the RNA hairpin from the clay to the silver aggregates is pH-dependent. Contact between adenine and the montmorillonite also seems to disperse adenine aggregates. The clay could also increase the flexibility of the RNA hairpin so that it is released from the clay at pH 10, and the affinity of its adenyl moieties for the metallic substrate is enhanced.

Defining 'life'

There is no broadly accepted definition of 'life.' Suggested definitions face problems, often in the form of robust counter-examples. Here we use insights from philosophical investigations into language to argue that defining 'life' currently poses a dilemma analogous to that faced by those hoping to define 'water' before the existence of molecular theory. In the absence of an analogous theory of the nature of living systems, interminable controversy over the definition of life is inescapable.

A highly variable segment of human subterminal 16p reveals a history of population growth for modern humans outstide Africa

We have sequenced a highly polymorphic subterminal noncoding region from human chromosome 16p13.3 flanking the 5' end of the hypervariable minisatellite MS205, in 100 chromosomes sampled from different African and Euroasiatic populations. Coalescence analysis indicates that the time to the most recent common ancestor (approximately 1 million years) predates the appearance of anatomically modern human forms. The root of the network describing this variability lies in Africa. African populations show a greater level of diversity and deeper branches. Most Euroasiatic variability seems to have been generated after a recent out-of-Africa range expansion. A history of population growth is the most likely scenario for the Euroasiatic populations. This pattern of nuclear variability can be reconciled with inferences based on mitochondrial DNA.